Crystal Hardness

by Michael Sandler

In general, strength and hardness are not synonymous terms. Material strength can be evaluated on a number of criteria.....tensile strength, hardness, brittleness, etc. Dependent on what your criteria are for defining "strength" in the case of a metal....your answers will be different. In terms of hardness, tungsten carbide is the probably hardest material currently used in the manufacturing of watch cases. Rado, Movado, and a number of Asian manufacturers are working with this material. Although extremely scratch may be more prone to stress fractures than steel in certain cases.

I decided to see what I could find on the subject of material hardness as it applies to scratch resistance. In most cases, the term hardness/strength is used somewhat synonymously to scratch resistance.

From what I have been able to find.....sapphire rates as a 9 on the Mohs Hardness Scale, which is a commonly used hardness ranking.

Below is a revised Mohs' Hardness Scale with some everyday items listed:
Gypsum (fingernail at 2.5)
Calcite (copper - old penny at 3.5)
Apatite (window glass or typical knife blade at under 5.5)
Orthoclase (good steel file at over 6.5)
Corundum (sapphire, ruby)

The Mohs' Hardness Scale is relative, which means that fluorite at 4 is not twice as hard as gypsum at 2; nor is the difference between calcite and fluorite similar to the difference between corundum and diamond. An absolute hardness scale is somewhat different than the relative scale. Using special equipment (which I do not really understand), a comparison of the absolute hardness of minerals can be measured. It turns out that most minerals are close in hardness. But as hardness increases, the difference in hardness greatly increases as seen in the scale below. Please note that the actual values of the hardnesses have no units of measure....they are only useful when compared against one another.

This is an absolute hardness scale:

As you can see, diamond is 4 times harder than sapphire, even though the two are only separated by a single point on the Mohs scale. Another important note is that many minerals display different hardnesses based on the direction of the stress, and thus will actually behave as softer or harder depending on the stress applied. Corundum (sapphire) is not one of these minerals.

A variation of the aforementioned absolute scale is the Knoop hardness scale. This absolute hardness scale provides a better method for direct comparison of two materials. Sapphire has an absolute hardness of between 2000 and 2050. Generally speaking, any material will scratch another material that is lower on the scale.

Both ruby and sapphire are varieties of corundum (see above) Some synthetic materials which have hardnesses of 9 (and thus would scratch a sapphire crystal) or above are as follows:
Synthetic (or natural) diamond: 10 (Absolute 8000 to 8500)
Tungsten carbide: 9.0 (Rado uses this in some watch cases, as does Movado). The material is very heavy, with density of about 15.7g/cubic centimeter. On the absolute hardness scale, it ranks as very slightly harder than sapphire (Absolute 2050 to 2150).
Silicon Carbide: About 9.4 (I don't think this is used in watches...generally used as an abrasive. Absolute 2150 to 2950)
Boron Carbide: About 9.5 (I don't think this is used in watches. Absolute 2900 to 3900)
Cubic Boron Nitride: In excess of 9.9 (Not used in watches. Very difficult to manufacture, and can only be made in very small pieces. Absolute 4500 to 4600) .

Most of these materials are not commonly occurring in everyday life, with the exception of perhaps diamond. It is not likely that a sapphire crystal will become very scratched.

It is important to note that in the literal sense, hardness is not synonymous with strength. Brittleness basically indicates how resistant the material is to plastic deformation. A very brittle material will, when placed under stress, break/fracture rather than bend. In the case of a sapphire crystal versus a glass crystal, the sapphire is considerably more brittle. As a result, a sapphire crystal is more likely to chip or crack than is glass counterpart if both are subjected to an equally hostile stress (banging, etc.).

I hope this helps...and that is accurate. I am not an my understanding of this subject is a bit limited.


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